Conventionally, a lead-tin alloy solder plating solution made of an aqueous solution, which contains: at least one selected from an acid and a salt thereof; a soluble lead compound; a soluble tin compound; a nonionic surfactant; and a formalin condensate of naphthalenesulfonic acid or a salt thereof, is disclosed (for example, see Patent Literature 1 (PTL 1)). The plating solution contains, as an additive, the formalin condensate of naphthalenesulfonic acid or the salt thereof in an amount of 0.02 to 1.50 mass % with respect to lead ions. PTL 1 discloses that it is possible to form a lead-tin alloy protruding electrode, which has a small variation in the height of the surface; is smooth; and has a small variation in composition ratio of lead/tin even when plating is performed with this plating solution at a high current density.
In addition, a plating bath of tin or tin alloy, which includes: (A) a soluble salt made of any one of a tin salt and a mixture of a tin salt and a predetermined metal salt of silver, copper, bismuth and lead; (B) an acid or a salt of the acid; and (C) a specific phenanthroline-dione compound, is disclosed (for example, see Patent Literature 2 (PTL 2)). PTL 2 discloses that, by using the plating path, it is possible to have excellent uniform electrodepositivity and film appearance in the wide current density range and to obtain a uniform synthetic composition in the wide current density range.
Furthermore, a tin plating solution, which includes a tin ion source; at least one kind of a nonionic surfactant; and imidazoline-dicarboxylate and 1,10-phenanthroline as additives, is disclosed (for example, see Patent Literature 3 (PTL 3)). PTL 3 discloses that, by using the tin plating solution: there is no burning of plating; and excellent uniformity of the in-plane film thickness distribution is obtained, in plating of a highly-complexed printed board. Moreover, excellent uniformity of through hole plating is obtained.